In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA≧0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE, Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
Several problems arise when a resist film is exposed in the presence of water. For example, the acid once generated from a photoacid generator and a basic compound added to the resist can be partially leached in water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects.
These drawbacks of the ArF immersion lithography may be overcome by providing a protective coating between the resist film and water to prevent resist components from being leached out and water from penetrating into the resist film (see the 2nd Immersion Workshop, Resist and Cover Material Investigation for Immersion Lithography, 2003).
With respect to the protective coating on the photoresist film, a typical antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCs are made of fluorinated compounds having a low refractive index, such as perfluoroalkyl polyethers and perfluoroalkyl amines. Since these fluorinated compounds are less compatible with organic substances, fluorocarbon solvents are used in coating and stripping of protective coatings, raising environmental and cost issues.
Other resist protective coating materials under investigation include water-soluble or alkali-soluble materials. See, for example, JP-A 6-273926, Japanese Patent No. 2,803,549, and J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 615, 2005. Since the alkali-soluble resist protective coating material is strippable with an alkaline developer, it eliminates a need for an extra stripping unit and offers a great cost saving. From this standpoint, great efforts have been devoted to develop water-insoluble, alkali-soluble resist protective coating materials, for example, methacrylate resins having fluorinated alcohol on side chain.
Effective means for preventing resist components from being leached out and water from penetrating into the resist film without a need for a protective coating material is proposed in JP-A 2006-48029 as adding an alkali-soluble, hydrophobic compound to the resist material. This method achieves equivalent effects to the use of resist protective coating material because the hydrophobic compound is segregated at the resist surface during resist film formation. Additionally, this method is economically advantageous over the use of a resist protective film because steps of forming and stripping the protective film are unnecessary.
Required for the resist protective coating materials or hydrophobic resist additives are not only the ability to prevent the generated acid and basic compound in the photoresist film from being leached out in water and to prevent water from penetrating into the resist film, but also such properties as water repellency and water slip. Of these properties, water repellency is improved by introducing fluorine into the resin and water slip is improved by combining water repellent groups of different species to form a micro-domain structure, as reported, for example, in XXIV FATIPEC Congress Book, Vol. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97 (1997).
Although the introduction of fluorine into resins is effective not only for improving water repellency, but also for improving water slip properties as demonstrated by sliding angle, receding contact angle or the like, excessive introduction of fluorine results in resins with a greater surface contact angle following alkaline development, which in turn invites an increased likelihood that development defects so called “blob defects” occur. Use of a more hydrophilic resist protective coating controls blob defects, but provides a smaller receding contact angle, which interferes with high-speed scanning and allows water droplets to remain after scanning, giving rise to defects known as water marks. There exists a demand for a resist protective coating material and a hydrophobic resist additive which have sufficient properties to prevent resist components from being leached out and to become a barrier to water, while maintaining a greater receding contact angle.
One exemplary polymer exhibiting high water slip is a fluorinated ring-closing polymerization polymer having hexafluoroalcohol pendants. It is reported in Proc. SPIE, Vol. 6519, p 651905 (2007) that this polymer is further improved in water slip by protecting hydroxyl groups on its side chains with acid labile groups. Use of the polymer having hydroxyl groups protected as a resist additive provides higher water slip than a topcoat of a similar polymer having hydroxyl groups unprotected.
The materials discussed above are needed not only in the ArF immersion lithography, but also in the electron beam (EB) lithography. The resist undergoes changes in sensitivity during EB lithography for mask image writing or the like. The resist sensitivity changes due to evaporation of the acid generated during image writing, evaporation of vinyl ether produced by deprotection of acetal protective groups, or the like, as discussed in JP-A 2002-99090. It is proposed to suppress resist sensitivity variation by applying a protective coating material or a hydrophobic additive-containing resist material to form a barrier film on top of a resist layer.